Cantilevered Wood Beams
Cantilevered Wood Beams
(OP)
Where in the building code, ASCE 7-10 or the AWC NDS does it specify the min. backspan for wood beams? From somewhere I have the 1/3 cantilever rule but I need to nail this down.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com






RE: Cantilevered Wood Beams
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RE: Cantilevered Wood Beams
RE: Cantilevered Wood Beams
I think back spans are called out/specified in prescriptive codes like the IRC, because many nail benders would put the fixed end of a canti. in a joist hanger and if it didn’t immediately fall down, call er good. Seeing it in the IRC and some product lit. is for self protection, liability protection, because many builders just don’t seem to know any better. Most codes allow engineers some latitude because they should know better, and actually be able to design the back span or the moment connection, to fit the actual conditions and their needs.
RE: Cantilevered Wood Beams
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Wood Beams
RE: Cantilevered Wood Beams
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Cantilevered Wood Beams
RE: Cantilevered Wood Beams
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Wood Beams
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Cantilevered Wood Beams
First, to address the question, IBC 2308.12.6.1.2, references the 2:1 back span. It MAY be elsewhere.
(Wisconsin is neither IBC 2015 nor ASCE 7-10 yet, so, I'm not there either.)
Notwithstanding the entire "Irregular structures" portion of 2308.12, prior to the "back span" reference, it is stated, "Such irregular portions of structures shall be designed to resist the forces specified in Chapter 16 to the extent such irregular features affect the performance of the conventional framing system." This sounds like an "empirical" statement putting the ultimate responsibility and freedom on the EOR.
I believe this agrees with JAE and specifically the last sentence of dhengr.
But, why do you "need to nail this down"? (assuming no pun intended)
Is a code reviewer or inspector requiring or requesting this of you?
I don't have a specific example, but, i KNOW I've (intentionally) "violated" the 1/3 or 2:1 "rule-of-thumb" in necessary situations. I believe i've even had less of a back-span than the cantilever; therefore, the "back-end" connection is critical to resist the reaction.
RE: Cantilevered Wood Beams
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Cantilevered Wood Beams
Depends. If the attic trusses don't cantilever, other than the top chord overhang, then you could just cut them short 3.5" and hanger them from the beam. You incur the cost of the hangers but then you have something positive to control that unwanted hinging that I mentioned. With the dropped header, I don't see how that gets resolved.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Wood Beams
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com